The following commit has been merged into the sched/core branch of tip:

Commit-ID:     15ad45fb80ca7fe67faf6b51dffce125a801cc5a
Gitweb:        https://git.kernel.org/tip/15ad45fb80ca7fe67faf6b51dffce125a801cc5a
Author:        Tim Chen <tim.c.chen@linux.intel.com>
AuthorDate:    Wed, 01 Apr 2026 14:52:22 -07:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 09 Apr 2026 15:49:50 +02:00

sched/cache: Count tasks prefering destination LLC in a sched group

During LLC load balancing, tabulate the number of tasks on each runqueue
that prefer the LLC contains the env->dst_cpu in a sched group.

For example, consider a system with 4 LLC sched groups (LLC0 to LLC3)
balancing towards LLC3. LLC0 has 3 tasks preferring LLC3, LLC1 has
2, and LLC2 has 1. LLC0, having the most tasks preferring LLC3, is
selected as the busiest source to pick tasks from.

Within a source LLC, the total number of tasks preferring a destination
LLC is computed by summing counts across all CPUs in that LLC. For
instance, if LLC0 has CPU0 with 2 tasks and CPU1 with 1 task preferring
LLC3, the total for LLC0 is 3.

These statistics allow the load balancer to choose tasks from source
sched groups that best match their preferred LLCs.

Co-developed-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/3d8502a33a753c4384b368f97f64ee70b1cea0db.1775065312.git.tim.c.chen@linux.intel.com
---
 kernel/sched/fair.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7d52cf0..cea625c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -10834,6 +10834,9 @@ struct sg_lb_stats {
 	unsigned int nr_numa_running;
 	unsigned int nr_preferred_running;
 #endif
+#ifdef CONFIG_SCHED_CACHE
+	unsigned int nr_pref_dst_llc;
+#endif
 };
 
 /*
@@ -11328,6 +11331,20 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 		if (cpu_overutilized(i))
 			sgs->group_overutilized = 1;
 
+#ifdef CONFIG_SCHED_CACHE
+		if (sched_cache_enabled()) {
+			struct sched_domain *sd_tmp;
+			int dst_llc;
+
+			dst_llc = llc_id(env->dst_cpu);
+			if (llc_id(i) != dst_llc) {
+				sd_tmp = rcu_dereference_all(rq->sd);
+				if (sd_tmp && (unsigned int)dst_llc < sd_tmp->llc_max)
+					sgs->nr_pref_dst_llc += sd_tmp->llc_counts[dst_llc];
+			}
+		}
+#endif
+
 		/*
 		 * No need to call idle_cpu() if nr_running is not 0
 		 */